阿飞Javaer，转载请注明原创出处，谢谢！

源码分析

位于sharding-jdbc-core模块下的包com.dangdang.ddframe.rdb.sharding.hint中，核心类HintManagerHolder的部分源码如下：

/**
 * Hint manager holder.
 * <p>Use thread-local to manage hint.</p>
 * @author zhangliang
 */
@NoArgsConstructor(access = AccessLevel.PRIVATE)
public final class HintManagerHolder {
    
    // hint特性保存数据的核心变量，即保存一个HintManager类型对象到ThreadLocal中
    private static final ThreadLocal<HintManager> HINT_MANAGER_HOLDER = new ThreadLocal<>();
    
    /**
     * Set hint manager.
     * @param hintManager hint manager instance
     */
    public static void setHintManager(final HintManager hintManager) {
        Preconditions.checkState(null == HINT_MANAGER_HOLDER.get(), "HintManagerHolder has previous value, please clear first.");
        HINT_MANAGER_HOLDER.set(hintManager);
    }
    
    public static boolean isUseShardingHint() {
        // 判断当前线程中是否使用了sharding hint--即HintManager中的shardingHint为true
        return null != HINT_MANAGER_HOLDER.get() && HINT_MANAGER_HOLDER.get().isShardingHint();
    }
    
    public static Optional<ShardingValue<?>> getDatabaseShardingValue(final ShardingKey shardingKey) {
        // 如果使用了sharding hint，那么从ThreadLocal中取数据库的sharding值
        return isUseShardingHint() ? Optional.<ShardingValue<?>>fromNullable(HINT_MANAGER_HOLDER.get().getDatabaseShardingValue(shardingKey)) : Optional.<ShardingValue<?>>absent();
    }
    
    public static Optional<ShardingValue<?>> getTableShardingValue(final ShardingKey shardingKey) {
        // 如果使用了sharding hint，那么从ThreadLocal中取表的sharding值
        return isUseShardingHint() ? Optional.<ShardingValue<?>>fromNullable(HINT_MANAGER_HOLDER.get().getTableShardingValue(shardingKey)) : Optional.<ShardingValue<?>>absent();
    }
    
    public static boolean isMasterRouteOnly() {
        // 是否强制路由主库--sharding-jdbc的特性之一：强制路由
        return null != HINT_MANAGER_HOLDER.get() && HINT_MANAGER_HOLDER.get().isMasterRouteOnly();
    }
    
    public static boolean isDatabaseShardingOnly() {
        // 是否只是数据库sharding
        return null != HINT_MANAGER_HOLDER.get() && HINT_MANAGER_HOLDER.get().isDatabaseShardingOnly();
    }
    
    /**
     * Clear hint manager for current thread-local.
     */
    public static void clear() {
        // ThreadLocal用完需要清理
        HINT_MANAGER_HOLDER.remove();
    }
}

ThreadLocal中管理的HintManager定义如下：

@NoArgsConstructor(access = AccessLevel.PRIVATE)
public final class HintManager implements AutoCloseable {
    
    // 数据库强制路由的值
    private final Map<ShardingKey, ShardingValue<?>> databaseShardingValues = new HashMap<>();
    
    // 表强制路由的值
    private final Map<ShardingKey, ShardingValue<?>> tableShardingValues = new HashMap<>();
    
    // 即是否使用了强制路由特性
    @Getter
    private boolean shardingHint;
    
    // 是否强制路由到主数据库
    @Getter
    private boolean masterRouteOnly;
       
    @Getter
    private boolean databaseShardingOnly;

    ... ...
    
    @Override
    public void close() {
        HintManagerHolder.clear();
    }
}

sharding值保存在ThreadLocal中，所以需要在操作结束时调用HintManager.close()来清除ThreadLocal中的内容。HintManager实现了AutoCloseable接口，推荐使用try with resource（JDK7新特性，参考Java 7中的Try-with-resources）自动关闭清理ThreadLocl线程中的数据。

如何使用

分析了sharding-jdbc的强制路由实现的源码，接下来说说如何使用这一niubility特性，假定数据源定义如下：

private static ShardingDataSource getShardingDataSource() throws SQLException {
    DataSourceRule dataSourceRule = new DataSourceRule(createDataSourceMap());
    TableRule orderTableRule = TableRule
            .builder("t_order")
            .actualTables(Arrays.asList("t_order_0", "t_order_1"))
            .dataSourceRule(dataSourceRule)
            .build();
    TableRule orderItemTableRule = TableRule
            .builder("t_order_item")
            .actualTables(Arrays.asList("t_order_item_0", "t_order_item_1"))
            .dataSourceRule(dataSourceRule)
            .build();
    ShardingRule shardingRule = ShardingRule.builder()
            .dataSourceRule(dataSourceRule)
            .tableRules(Arrays.asList(orderTableRule, orderItemTableRule))
            .bindingTableRules(Collections.singletonList(new BindingTableRule(Arrays.asList(orderTableRule, orderItemTableRule))))
            .databaseShardingStrategy(new DatabaseShardingStrategy("user_id", new ModuloDatabaseShardingAlgorithm()))
            .tableShardingStrategy(new TableShardingStrategy("order_id", new ModuloTableShardingAlgorithm())).build();
    return new ShardingDataSource(shardingRule);
}

根据数据源定义可知，数据库的sharding column为user_id，表的sharding column为order_id；

1、强制路由数据库

• 如何使用

private static void printHintSimpleSelect(final DataSource dataSource) throws SQLException {
    // SQL语句并不涉及任何数据库路由和表路由信息（即where语句中没有user_id条件和order_id条件）
    String sql = "SELECT i.* FROM t_order o JOIN t_order_item i ON o.order_id=i.order_id";
    try (
            HintManager hintManager = HintManager.getInstance();
            Connection conn = dataSource.getConnection();
            PreparedStatement preparedStatement = conn.prepareStatement(sql)) {
        // 强制路由：数据库路由sharding column即user_id的值为10
        hintManager.addDatabaseShardingValue("t_order", "user_id", 10);
        try (ResultSet rs = preparedStatement.executeQuery()) {
            //todo do something
        }
    }
}

由于指定了强制路由数据库的值user_id=10，所以只会输出ds_jdbc_0这个库中符合条件的数据。而ds_jdbc_1会被过滤；

• 实现原理

private Collection<String> routeDataSources(final TableRule tableRule) {
    // 首先得到数据库sharding策略，例如：数据库按照列user_id进行sharding
    DatabaseShardingStrategy strategy = shardingRule.getDatabaseShardingStrategy(tableRule);
    // 然后从ThreadLocal中取出sharding的值
    List<ShardingValue<?>> shardingValues = HintManagerHolder.isUseShardingHint() ? getDatabaseShardingValuesFromHint(strategy.getShardingColumns())
            : getShardingValues(strategy.getShardingColumns());
    Collection<String> result = strategy.doStaticSharding(tableRule.getActualDatasourceNames(), shardingValues);
    Preconditions.checkState(!result.isEmpty(), "no database route info");
    return result;
}

2、强制路由表

• 如何使用

private static void printHintSimpleSelect(final DataSource dataSource) throws SQLException {
    // SQL语句并不涉及任何数据库路由和表路由信息（即where语句中没有user_id条件和order_id条件）
    String sql = "SELECT i.* FROM t_order o JOIN t_order_item i ON o.order_id=i.order_id";
    try (
            HintManager hintManager = HintManager.getInstance();
            Connection conn = dataSource.getConnection();
            PreparedStatement preparedStatement = conn.prepareStatement(sql)) {
        // 强制路由：表路由sharding column即order_id的值为1000
        hintManager.addTableShardingValue("t_order", "order_id", 1000);
        try (ResultSet rs = preparedStatement.executeQuery()) {
            //todo do something
        }
    }
}

由于指定了强制路由表的值order_id=1000，所以只会输出所有库中与t_order_0 匹配的数据。而与t_order_1匹配的数据会被过滤；

• 实现原理

private Collection<String> routeTables(final TableRule tableRule, final String routedDataSource) {
    // 首先得到表的sharding策略，例如：表按照列order_id进行sharding
    TableShardingStrategy strategy = shardingRule.getTableShardingStrategy(tableRule);
    // 然后从ThreadLocal中取出sharding的值
    List<ShardingValue<?>> shardingValues = HintManagerHolder.isUseShardingHint() ? getTableShardingValuesFromHint(strategy.getShardingColumns())
            : getShardingValues(strategy.getShardingColumns());
    Collection<String> result = tableRule.isDynamic() ? strategy.doDynamicSharding(shardingValues) : strategy.doStaticSharding(tableRule.getActualTableNames(routedDataSource), shardingValues);
    Preconditions.checkState(!result.isEmpty(), "no table route info");
    return result;
}

3、强制路由主库

• 如何使用

HintManager hintManager = HintManager.getInstance();
hintManager.setMasterRouteOnly();